linux-kernel - Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87366bbe1y.fsf@vitty.brq.redhat.com>
Date:   Wed, 01 Jul 2020 14:44:09 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit

Xiaoyao Li <xiaoyao.li@...el.com> writes:

> On 7/1/2020 5:04 PM, Vitaly Kuznetsov wrote:
>> Chenyi Qiang <chenyi.qiang@...el.com> writes:
> [...]
>>>   static const int kvm_vmx_max_exit_handlers =
>>> @@ -6830,6 +6838,13 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>>   	if (unlikely(vmx->exit_reason.failed_vmentry))
>>>   		return EXIT_FASTPATH_NONE;
>>>   
>>> +	/*
>>> +	 * check the exit_reason to see if there is a bus lock
>>> +	 * happened in guest.
>>> +	 */
>>> +	if (vmx->exit_reason.bus_lock_detected)
>>> +		handle_bus_lock(vcpu);
>> 
>> In case the ultimate goal is to have an exit to userspace on bus lock,
>
> I don't think we will need an exit to userspace on bus lock. See below.
>
>> the two ways to reach handle_bus_lock() are very different: in case
>> we're handling EXIT_REASON_BUS_LOCK we can easily drop to userspace by
>> returning 0 but what are we going to do in case of
>> exit_reason.bus_lock_detected? The 'higher priority VM exit' may require
>> exit to userspace too. So what's the plan? Maybe we can ignore the case
>> when we're exiting to userspace for some other reason as this is slow
>> already and force the exit otherwise? 
>
>> And should we actually introduce
>> the KVM_EXIT_BUS_LOCK and a capability to enable it here?
>> 
>
> Introducing KVM_EXIT_BUS_LOCK maybe help nothing. No matter 
> EXIT_REASON_BUS_LOCK or exit_reason.bus_lock_detected, the bus lock has 
> already happened. Exit to userspace cannot prevent bus lock, so what 
> userspace can do is recording and counting as what this patch does in 
> vcpu->stat.bus_locks.

Exiting to userspace would allow to implement custom 'throttling'
policies to mitigate the 'noisy neighbour' problem. The simplest would
be to just inject some sleep time.

-- 
Vitaly